Improve shared_alloc_shm.c strategy

Try allocating a contiguous buffer first, and only then use segmentation.
Segmentation breaks the JIT, so a contiguous buffer is preferable.

Author: iluuu1994

ext/opcache/shared_alloc_shm.c

@@ -42,7 +42,6 @@
 # define MIN(x, y) ((x) > (y)? (y) : (x))
 #endif
 
-#define SEG_ALLOC_SIZE_MAX 64*1024*1024 /* 64MB to match ZEND_JIT_DEFAULT_BUFFER_SIZE */
 #define SEG_ALLOC_SIZE_MIN 2*1024*1024
 
 typedef struct  {
@@ -53,36 +52,38 @@ typedef struct  {
 static int create_segments(size_t requested_size, zend_shared_segment_shm ***shared_segments_p, int *shared_segments_count, const char **error_in)
 {
 	int i;
-	size_t allocate_size = 0, remaining_bytes = requested_size, seg_allocate_size;
+	size_t allocate_size = 0, remaining_bytes, seg_allocate_size;
 	int first_segment_id = -1;
 	key_t first_segment_key = -1;
 	struct shmid_ds sds;
 	int shmget_flags;
 	zend_shared_segment_shm *shared_segments;
 
-	seg_allocate_size = SEG_ALLOC_SIZE_MAX;
-	/* determine segment size we _really_ need:
-	 * no more than to include requested_size
-	 */
-	while (requested_size * 2 <= seg_allocate_size && seg_allocate_size > SEG_ALLOC_SIZE_MIN) {
-		seg_allocate_size >>= 1;
-	}
-
 	shmget_flags = IPC_CREAT|SHM_R|SHM_W|IPC_EXCL;
 
-	/* try allocating this much, if not - try shrinking */
-	while (seg_allocate_size >= SEG_ALLOC_SIZE_MIN) {
-		allocate_size = MIN(requested_size, seg_allocate_size);
-		first_segment_id = shmget(first_segment_key, allocate_size, shmget_flags);
-		if (first_segment_id != -1) {
-			break;
+	/* Try contiguous allocation first. */
+	seg_allocate_size = requested_size;
+	first_segment_id = shmget(first_segment_key, seg_allocate_size, shmget_flags);
+	if (UNEXPECTED(first_segment_id == -1)) {
+		/* Search for power of n^2 < requested_size. */
+		seg_allocate_size = 1024 * 1024;
+		while (seg_allocate_size < requested_size / 2) {
+			seg_allocate_size *= 2;
 		}
-		seg_allocate_size >>= 1; /* shrink the allocated block */
-	}
 
-	if (first_segment_id == -1) {
-		*error_in = "shmget";
-		return ALLOC_FAILURE;
+		/* try allocating this much, if not - try shrinking */
+		while (seg_allocate_size >= SEG_ALLOC_SIZE_MIN) {
+			first_segment_id = shmget(first_segment_key, seg_allocate_size, shmget_flags);
+			if (first_segment_id != -1) {
+				break;
+			}
+			seg_allocate_size >>= 1; /* shrink the allocated block */
+		}
+
+		if (first_segment_id == -1) {
+			*error_in = "shmget";
+			return ALLOC_FAILURE;
+		}
 	}
 
 	*shared_segments_count = ((requested_size - 1) / seg_allocate_size) + 1;